FDR certainly took advantage of the progressive movement but the huge electoral block was also built on reuniting southern and northern democrats - a bridge that may no longer be able to be crossed.

I am not sure I understand the last few paragraphs - Truman vetoed the Taft-Hartley Act, his policies were described at the time as "a grab bag of well worn New Deal ideas", and routinely sttacked the "gluttons of power" and "Wall Street reactionaires" (his words).  McCarthyism didn't start until Truman's second term, post election. The distinction between Wallace, who favored socialism, and Truman is more than what you describe.

As for Truman not being a New Dealer - his Senate record shows otherwise.  He voted with Roosevelt on every piece of legislation, except possiblely expansion of the court, and was routinely mocked in Missouri as a New Deal "yes-man" so much so that he faced a serious challange to re-election to his senate seat in 1940.